Optical communication systems are rapidly becoming of increased and military importance. These systems convert an electrical signal to a modulated light signal using a light emitting diode or an injection laser. This light signal is transmitted along a plastic or glass optical fiber to a light detector such as a photodiode where the light signal is reconverted to an electrical signal. The highest bandwidth optical communication systems use an injection laser as a light source, a single mode fused silica optical fiber as the transmission medium, and an avalanche or p-i-n photodiode as the light detector. Single mode optical fibers are required in such systems to eliminate the dispersion resulting in multimode optical fibers from the different path lengths for the different simultaneously propagating modes. One class of single mode optical fibers is comprised of a doped fused silica core, typically about 5 microns in diameter, surrounded by a first cladding of borosilicate glass about 22 microns thick and a second cladding of fused silica, typically about 15 microns thick. A protective coating of an organic material, typically 10 to 15 microns thick, surrounds the cladding. A second class of single mode optical fiber is comprised of a doped fused silica core about 5 microns in diameter surrounded by a fused silica cladding about 35 microns thick.
The borosilicate glass cladding has an optical index of refraction at the wavelength of the propagating light which is less than that of the core. Light rays which strike the interface between the core and the first cladding at an angle of incidence less than the critical angle for total internal reflection are partially transmitted through the interface and are lost; while those that are incident at an angle greater than the critical angle are totally reflected at this interface and propagate along the fiber. The energy in the light beam is primarily concentrated in the core with the remainder propagating as an evanescent wave in the cladding. Single mode optical propagation in an optical fiber is obtained by reducing the diameter of the fiber core until only a single mode can propagate and all other modes are lost by transmission through the interface.
The amount of light which can be coupled into the optical fiber is limited by the light source size and the angular distribution of its output and by the area and acceptance angle or numerical aperture of the fiber core. To improve the coupling of light into the fiber it has been found to be useful to form a small f-number positive lens directly on the end of the optical fiber. Pan et al, U.S. Pat. No. 4,118,270 issued Oct. 3, 1978, incorporated herein by reference, have disclosed two methods for forming a positive lens on the end of an optical fiber. The first method is to form a hemispherical end on the fiber by etching the end in hydrofluoric acid. The second method is to dip a flat fiber end into a light transmissive epoxy resin, allowing surface tension and gravity to form a drop of the desired shape, either a hemispherical or hyperbolic shape. Upon hardening of the epoxy resin the desired lens is formed.
Either of these approaches results in a small f-number positive lens and will provide improved light coupling efficiency for multimode optical fibers with a large core diameter and a small cladding thickness. For the converse case of a single mode optical fiber where the core diameter is small and the cladding thickness is large, a large f-number lens results using this process and the resultant improvement in light coupling is much smaller. This result occurs because the radius of curvature of the lens formed and, thus, its focal length, is determined principally by the diameter of the large cladding region and not by the active core region into which it is desired to couple the light.
Timmerman, Applied Optics, Vol. 15, page 2432 (1976), has disclosed a three step technique of etching the end of the fiber to reduce its overall diameter, cutting the fiber end to provide a flat surface at the fiber end and positioning a low melting temperature lens on the fiber end. Reducing the diameter of the fiber end increases the radius of curvature of the lens formed, thus decreasing the focal length and the f-number of the lens formed by the technique of Pan et al. This significantly increases the efficiency of coupling of light into the single mode fiber.
Cohen et al, U.S. Pat. No. 3,883,353 issued May 13, 1975, have disclosed a photolithographic method to shape the ends of two cladded core fibers to form a high efficiency male to female connection between them. The portion of the fiber end to be etched is defined by using standard photolithographic techniques and an etchant such as hydrofluoric acid to remove the undesired material.